Turbojet engines develop thrust by expanding a flow of hot gas at high velocity through an outlet at the aft end of the engine. This flow of gas, known as the jet exhaust, flows from the engine outlet at considerable velocity, and this velocity and the mass flow of the jet exhaust produces the forward thrust of the engine. The thrust thus is a function of the velocity and mass flow of the exhaust gases.
Because the jet exhaust requires a relatively high velocity to produce the desired thrust, the noise created by the jet exhaust is loud and objectionable at close range. People living near airports receive the brunt of jet exhaust noise because turbojet engines during takeoff are operating at maximum power to achieve safe takeoff and climbout of the aircraft. Some communities have passed laws prohibiting late-night flights at airports and restricting flight operations at other times, in an effort to reduce the perceived noise on the ground.
The design of turbojet engines has evolved in efforts to reduce the noise created by the jet exhaust. For example, baffles have been attached to the engines surrounding the exhaust outlet in an effort to reduce the component of noise directed downwardly toward the ground. Another such development is the so-called turbofan engine, having an air bypass duct extending rearwardly from an air inlet near the forward end of the engine. The bypass duct extends aft and terminates at an outlet which generally surrounds the exhaust outlet of the engine. A fan is positioned in the inlet end of the bypass duct; this fan is driven by the existing turbine of the engine to produce significant airflow directed rearwardly through the bypass duct. This bypass air surrounds the jet exhaust at the aft end of the turbofan engine, and the bypass air mixes with the jet exhaust to reduce the noise-producing energy of that exhaust. The bypass fan of a turbofan engine also can produce significant amounts of thrust, particularly at relatively low forward velocity of the aircraft where jet engines are relatively inefficient at producing reaction thrust.
Modern turbofan engines generally produce much less jet exhaust noise than do conventional turbojet engines of comparable power. However, aircraft originally equipped with turbojet engines may be restricted or prevented from operating in communities with increasingly-strict noise standards, and those aircraft are not easily or economically modifiable to use turbofan engines. Converting an existing turbojet engine to a turbofan engine heretofore has required a substantial redesign and rebuild of the engine, an expedient which may cost more than the original engine. Replacing the existing turbojet engines with turbofan engines also is expensive. Moreover, such proposed engine swaps seldom are practical because of changes in weight, range, and other factors encountered when substituting engines other than the ones for which the aircraft was designed.